In a paper machine, sheet-shaped wet paper is formed from feed stock, and the wet paper is processed into product paper by removing moisture from the former.
As drying is an essential step for removing moisture, a so-called dry part where a drying process is carried out plays a very important role.
The paper machine is equipped with a plurality of dryers for drying the wet paper, occupying the major part of the paper machine.
The dryers normally have a construction such that the dryers can be heated from inside thereof by introducing heated steam and so forth thereinto.
When moist paper undried as yet is fed to the dry part, the paper is pressed into contact with the surface of the dryers by touch rolls and canvases, and dried.
The surface of the dryers made of metal is generally a rough surface in microscopic terms, and especially since dryers made of casting are in widespread use, it is unavoidable that the surface thereof has such roughness.
Incidentally, paper contains pitch, tar component, and microfibers that are included in pulp feed stock itself, additive chemicals contained in various papers, and other components such as filler. When the paper is pressed against the surface of the dryers, those components described tend to gain adhesiveness due to the effect of heat, and to stick to the surface of the dryers.
For removal of contaminants stuck to the surface of the dryers such as the components described above, there has been normally adopted a method of scraping the contaminants off with a doctor blade, an accessory of the dryers.
However, this causes the surface of the dryers to become rougher due to friction occurring between the doctor blade and the surface of the dryers, the components described above make ingress into recesses in microscopic aspesrities on the rough surface, and stick thereto under the influence of heat and pressure. Then, parts of the surface of the wet paper are transferred to the dryers, and scraped off again with the doctor blade. Thus, there will occur a vicious cycle of the same phenomena being repeated.
As described in the foregoing, since in the case of conventional methods of making paper, the components described above stick to the dryers, and concurrently, the surface structure of paper is stripped off, the method incurs direct or indirect adverse effects caused by the components.
For example, technical problems as described hereinafter will be encountered;
1. Paper powders generated are mixed with products, and especially at the time of printing, transfer of ink to the surface of paper is blocked by the paper powders, causing the phenomenon called “counter” to occur.
2. Causes for unevenness and napping, occurring on the surface of product paper, and degradation in the surface strength of the product paper are created.
3. Thermal conductivity of the surface of the dryers becomes lower, degrading a drying rate of paper.
4. The phenomenon called “picking” whereby the surface of paper is peeled off occurs.
5. There will be an increase in the number of periodical clean-ups required of the dryers.
6. Sticking of paper to the surface of the dryers occurs, resulting in breaks of paper.
Accordingly, attempts have been made to overcome shortcomings as described above as much as possible by applying chromium plating or Teflon coating to the surface of the dryers beforehand, or by applying sufficient oil hardening treatment thereto periodically while the paper machine is out of operation.
However, in the former case, after surface-treated dryers have been in use over time, the treated surface thereof undergoes gradual wear due to friction, resulting in degradation in the effect of contamination prevention.
In the case of degradation in the effect taking place, it is required that the dryers should be replaced with new ones, or the surface thereof is ground, resulting in loss in operation time due to time required for replacement, or extra costs incurred.
Similarly, in the latter case, transfer of oil to paper takes place over time, and as a result, the beneficial effects of oil starts to decline, so that there will be a limitation to the merits of this method.
Thus, the beneficial effects over the long term can not be expected of either of the methods described in the foregoing, and both the methods are therefore not suited for continuous operation on the long term basis.